Electron curing has been applied to preparation of supported cured magnetic coatings of certain polyurethane compositions and a magnetic pigment, for example, in the form of discs or tapes. These products are useful in video, audio and data processing equipment. Oligomeric polyurethanes terminated with unsaturated acrylate groups that form reactive radicals for curing and cross-linking under the influence of electron beam radiation are being increasingly used to replace the chemically cross-linked polyurethanes previously used.
Typical U.S. patents describing these acrylate terminated polyurethanes include: No. 4,260,703 that discloses radiation curable urethane-acrylates preparing by reacting an alkoxylate of an ester diol such as 2,2-diemthyl-3-hydroxypropyl-2,2-di-methyl-3-hydroxypropionate with a diisocyanate, capped with a hydroxyalkyl acrylate such as hydroxyethyl acrylate; No. 4,264,752 that discloses compositions formed by the reaction of a polycarbonate polyol, a polyisocyanate and a hydroxy acrylate monomer as hydroxyethyl acrylate that can be used as radiation curable coatings; No. 4,408,020 that discloses the reaction products of a hydroxyl terminated polymer, unsaturated polyethers containing at least one terminal double bond group and a diisocyanate; and No. 4,446,286 that discloses reaction products of a hydroxyl terminated polymer, a terminally unsaturated polyether and an organic diisocyanate, which is then mixed with an acrylate terminated polyurethane. These materials as a class are generally referred to as oligomeric polyurethanes. While they are cured by an electron beam to form useful magnetic tapes for example, they have disadvantages.
For example, a major problem is poor dispersibility of the magnetic pigment at high loadings, as about 75 to 80 weight percent, which is an essential requirement for the polyurethane binder in many applications such as the home video tapes. Polyurethanes that allow good dispersibility at high loadings and that are readily cured by electron radiation are among the objects of this invention.